The invention is grounded in the art of high-strength, high-modulus silicon carbide (SiC) monofilaments. Typically, these monofilaments exhibit tensile strengths in excess of 300,000 pounds per square inch (psi) and bending or Young's modulus in excess of 30 million psi.
The heretofore baseline SiC monofilament has a tensile strength of 350-400 thousand psi and a bending modulus of 55-60.times.10.sup.6 psi.
The field of technology defined as high-strength, high-modulus monofilaments is unique and ultracritical to changes in structure or process of manufacture.
Persons skilled in the art of high-strength, high-modulus filaments have observed that it is frequently not possible to predict what effect changes in compositions, processes, feedstocks, or post-treatments will have on the properties of this unique family of filaments.
Boron nitride, boron carbide, titanium nitride, titanium carbide, and tungsten in combination with boron or silicon carbide filaments have failed to provide a useful filament, though in each instance, the candidate material was chosen to enhance one or more properties of the high-strength filament.
A silicon carbide coating on a silicon carbide filament would not protect the filament from degradation unless the coating had a critical cross-section profile. See U.S. Pat. Nos. 4,340,636 and 4,415,609.
Carbon cores require buffer layers deposited at critical specific temperatures. See U.S. Pat. No. 4,142,008.
Carbon-rich silicon carbide outer layers which create and protect high tensile strengths or silicon carbide filaments were found to form ineffective metal matrix and resin matrix composite materials. See U.S. Pat. Nos. 4,340,636 and 4,415,609.
Recrystallization of fine grain structures occurs at one temperature. After 5 seconds of exposure to the temperature, the filament loses 50 percent of its strength. Exposure to a temperature in the order of 2 percent lower shows no subsequent degradation.
Critical crystal morphology of a carbon-rich region in a silicon carbide filament was found to improve machinability of silicon carbide reinforced composites. The proper claim structure was the difference between a commercially-viable monofilament and trash.
Other structural or manufacturing procedural features which were found to be of a critical nature are change in core composition and surface texture, the presence or absence of a buffer layer of specific compositions, impurities, and reactivity of a surface coating with matrix material.